Japanese Patent Application Laid-open Publication No. 2005-300596 (Patent Literature 1) describes a problem about the optical connector for connection between single mode fibers (each of which will be referred to hereinafter as “SMF”) that it is easily subjected to reduction in coupling efficiency due to misalignment of fiber axes for the reason of small core diameter. Furthermore, this Patent Literature 1 discloses a composite optical fiber for solving the foregoing problem. The composite optical fiber is configured by connecting a plurality of optical fibers of different core diameters in an increasing order of the core diameters. During passage of a beam through the composite optical fiber of this configuration, the beam diameter gradually increases with increase in core diameter. The beam diameter becomes maximum at a point of transfer of the beam between optical connectors. It is intended by this scheme that the reduction in coupling efficiency is suppressed even with occurrence of the misalignment between SMFs.
Japanese Patent No. 3259746 (Patent Literature 2) discloses an optical fiber array unit. This optical fiber array unit is composed of a plurality of subunits stacked. Each subunit has a first member which holds a plurality of SMFs in a state in which they are arranged in parallel to each other, a second member which holds graded-index optical fibers (each of which will be referred to hereinafter as “GRIN optical fiber”) for collimating beams output from the respective SMFs into parallel beams, and positioning means which positions the first member and the second member so as to coaxially connect the SMFs and GRIN optical fibers to each other.